This invention relates to a vehicle constant speed driving apparatus and, more particularly, to a vehicle constant speed driving apparatus for controlling the opening of a throttle valve by operating a hydraulic (vacuum) actuator having a modulator valve for adjusting pressure by a duty control, such as a rate of operating time.
Conventional modulator valves have had drawbacks that their capacity to adjust pressure varies in response to changes in the voltage of associated power sources and changes in the resistance of solenoids of the modulator valves thereby resulting in deviations in driving speeds. Changes in the voltage of a power source may be compensated for by maintaining at a constant value the voltage applied to a solenoid of a modulator valve, but it is difficult to compensate for the changes in the resistance of a solenoid resulting from changes in the temperature. The latter problem may be solved by maintaining at a constant value the current flowing through a solenoid of a modulator valve, but this is difficult in view of the inductance of the solenoid, because the ON-OFF repetition cycle of the solenoid is fast (ordinarily 20-80 Hz) in a valve system for adjusting pressure by the rate of operating time.